1. Field of the Invention
The present invention relates to radio frequency (RF) circuits implemented with multilayered low temperature co-fired ceramic (LTCC) substrates, and in particular, to RF filter circuits within LTCC substrates.
2. Description of the Related Art
As electronic circuitry has become increasingly sophisticated, many forms of device and circuit scaling have been used to reduce the size and space needed, as well as to provide durability. First, individual devices and other circuit components were mounted on printed circuit boards. As integrated circuits (ICs) became more developed, more devices and components became integrated, thereby allowing the printed circuit boards and other substrates to be further reduced in size. More recently, ICs have developed to the point where virtually entire systems are integrated within one die, or chip. However, one exception to this has been many forms of RF circuits due to the need for various capacitors and inductors which are difficult, if not impossible, to fully integrate within a chip. Accordingly, alternative techniques have been developed to miniaturize and provide durable circuits and subsystems. One technique has been the use of “hybrid” circuits in which ICs are mounted along with other forms of chip components, including chip resistors, inductors and capacitors, on some form of substrate (e.g., alumina ceramic) and then hermetically sealed for protection. Another technique which is seeing increased use is the use of multilayered LTCC substrates on which ICs and other chip components are mounted on the top surface, while passive components, such as inductors and capacitors, are formed among the underlying layers.
As is well known, a typical implementation of circuitry using an LTCC substrate includes multiple layers of a ceramic “tape” which are used to provide the base structure, i.e., substrate, within and upon which to form various electronic components and electrical connections. This tape is formed from a powdered ceramic material which is then mixed with a binder material. For example, one commonly used ceramic tape is that available from DuPont under the trade name “Green Tape 951”. Electronic components that can be formed within or among the various LTCC layers include resistors, capacitors and inductors. The electrical connections between each tape layer, similar to those connections formed within various layers within an integrated circuit, are known as “vias” and are formed by apertures lined or filled with a conductive material.
As is further well known, such components are formed by establishing (e.g., punching) holes in the tape as appropriate and layering metal, dielectric material and insulating material. Several layers of the tape are generally used to form the ultimate desired circuitry. These tape players are then pressed together and fired in an oven to remove the binder and sinter the ceramic powder. Components which are too large or difficult to include or form within or among the ceramic tape layers, e.g., IC chips, are typically surface-mounted on the top of the hardened substrate. The resulting substrate and components, often less than one inch square, provide a compact and durable packaged circuit.
Referring to FIG. 1, for example, integrated LTCC modules are frequently used in cellular wireless telephones. As discussed above, each of the multiple layers of ceramic material is printed with metallized circuit patterns that are electrically coupled layer-to-layer by conductive vias (small metallized holes which pass vertically through the ceramic material layers). The individual layers are then assembled, laminated under pressure and co-fired (fired as a unit) to create a monolithic structure. The external contacts may be plated with gold, nickel or tin to protect conductive metal and to facilitate interconnection at the next system level. The end result is a mechanically strong, hermetic, thermally conductive, chemically inert and dimensionally stable ceramic structure.
With respect to hermeticity, the internal conductors are protected by the surrounding dense ceramic material. Metal components, such as sealing rings, are brazed to the plated surface of the co-fired ceramic to provide protection of the surface-mounted IC dice. Having the electrical conductors buried within the ceramic structure reduces risks of short circuits due to environmental effects, such as moisture, dirt or other factors.
Circuit density is directly proportional to the number of layers. Increased density of the circuitry to be implemented may require the addition of layers to prevent undesirable electrical performance characteristics, such as crosstalk or other forms of electrical signal interference. Further, distributing circuitry on additional layers can help to avoid yield losses caused by very fine signal lines and spacings.
Some problems which can arise from such dense packaging, notwithstanding the use of additional layers, can be noise or signal-induced interference caused by close proximities of the various signal and power supply lines and changes in circuit performances due to the slight variations in alignment or registration between the various ceramic layers that can be expected within normal manufacturing tolerances. These issues, among others, are addressed by the presently claimed invention.